I was just researching quick ways to integrate a one of my latest Flipswap projects with Twitter when I ran across this obvious, yet non-obvious thing. You can construct a URL that pre-populates the update field with any text you like. It works like this (click it for full affect/effect):

http://twitter.com/home?status=Yeatblog+rocks+my+face+off.

The craziest thing about this “discovery” is that a search on Google for “post to twitter” yields exactly what you need to know in the results.